Assembly techniques for solar cell arrays and solar cells formed therefrom

ABSTRACT

An assembly technique for assembling solar cell arrays is provided. During the fabrication of a solar cell, openings through the semiconductor layer are etched through to a top surface of the backmetal layer. The solar cells include an exposed top surface of the backmetal layer. A plurality of solar cells are assembled into a solar cell array where adjacent cells are interconnected in an electrically serial or parallel fashion solely from the top surface of the solar cells.

RELATED APPLICATION

This application claims priority to Provisional Application No.61/175,259 filed on May 4, 2009, which is herein incorporated byreference in its entirety.

This invention was made with Government support under FA9453-09-C-0365awarded by Air Force Research Laboratory. The Government has certainrights in the invention.

TECHNICAL FIELD

The present application concerns photovoltaic devices, such as solarcell devices. More specifically, the present application concerns anassembly technique for forming solar cell arrays by interconnectingadjacent solar cells solely from one surface thereof.

BACKGROUND

A photovoltaic device converts light energy into electricity. Althoughthe term “solar cell device” may sometimes be used to refer to a devicethat captures energy from sunlight, the terms “solar cell device” and“photovoltaic device” are interchangeably used in the presentapplication regardless of the light source.

A conventional semiconductor solar cell 100 illustrated in FIG. 1includes three layers. The first layer 104 is formed by a metal grid onthe second layer. The second layer 106 is formed by a semiconductormaterial on the third layer. The third layer 108, i.e. the bottom orback layer, is a metal blanket layer.

Conventionally, adjacent solar cells are assembled into a solar cellarray using interconnect tabs connected to the front and back surfacesof the solar cells. FIGS. 2A-2D depict an exemplary solar cell array 200where interconnect tabs 206 are used to connect adjacent cells 100 in aserial fashion. The conventional method for assembling solar cells intoa solar cell array does not facilitate high-volume assembly of solarcell arrays because connections need to be made to both the back andfront surfaces of the solar cells as shown in FIGS. 2A-2D.

FIG. 2A illustrates a cross-sectional view of a conventional solar cellarray 200 assembled according to conventional methods. In particular,FIG. 2A illustrates the interconnect tabs 206 contacting the top andbottom surface of adjacent solar cells 100. As shown in FIG. 2A, theinterconnect tab 206 is connected to a bottom surface of the back layer108 of the first conventional solar cell 100 to form a bottom connection204. The interconnect tab 206 is also connected to a top surface of thesemiconductor layer 106 of an adjacent second solar cell 101 to form atop or front connection 202.

A bonding tool performs at least two sweeps across the solar cell arrayto perform the conventional front connection 202 and the back connection204 of the solar cells. The first sweep may be across the top or frontsurface of the semiconductor layer 106 of the solar cell array 200 forforming the front connections 202 illustrated in FIGS. 2A-2D. The secondsweep may be across the back or bottom surface of the back metal layer108 of the solar cell array 200 for forming the back connections 204illustrated in FIGS. 2A-2D.

FIG. 2B is a top view of the conventional solar cell array illustratedin FIG. 2A. As illustrated in FIG. 2B, the top surface of the firstsolar cell 100 is connected to the bottom surface of the adjacent secondsolar cell 101 via the interconnect tab 206. A portion of theinterconnect tab 206 is attached to the top surface of the first solarcell 100 to form the top or front connection 202. Another portion of theinterconnect tab 206 is attached to the bottom surface of the secondadjacent solar cell 101 to form the bottom connection 204.

FIG. 2C is a bottom view of the conventional solar cell arrayillustrated in FIG. 2A. As illustrated in FIG. 2B, in the conventionalmethod, a tab 208 for a bypass diode connection is provided on the topsurface of the semiconductor layer 106 of the conventional solar cell100. As illustrated in FIG. 2C, one surface of the bypass diode 209 isconnected to the back surface of the back metal layer 108 of theconventional same solar cell 100 while the opposite surface of thebypass diode 209 is connected to the aforementioned tab 208.

FIG. 2D is a side view close-up of an exemplary conventionalinterconnect tab 206 and the bypass diode tab 208 used in connectionwith the conventional solar cell array illustrated in FIGS. 2A-2C. Asillustrated in FIG. 2D, the bypass diode tab 208 effectively wrapsaround the edge of the solar cell 100 to contact the bypass diode 209 tothe top surface and effectively bypasses the solar cell when required.

Accordingly, a new method for assembling adjacent solar cells into asolar cell array is needed that facilitates high-volume assembly ofsolar cell arrays.

SUMMARY

Various embodiments of the present invention provide an assemblytechnique for assembling a plurality of solar cells into one or moresolar cell arrays. During the fabrication of a solar cell, openingsthrough the semiconductor layer are etched through to expose a topsurface of the backmetal layer. During the assembly process, whichfollows the fabrication process, adjacent cells in a solar cell arrayare interconnected in an electrically serial or parallel fashion solelyfrom one surface, e.g. the top surface, of the solar cells. The assemblymethod of the present invention and the solar cell structures formedusing the assembly method enable manufacturing high-volume, automatedassembly of solar cell array sheets.

In accordance with various embodiments of the present invention, amethod for interconnecting a first solar cell adjacent to a second solarcell in an array of solar cells is provided. The method includesproviding a first solar cell adjacent to a second solar cell. At leastthe first solar cell includes a semiconductor layer formed on top of abackmetal layer. The method also includes etching a portion of thesemiconductor layer of the first solar cell to expose a portion of a topsurface of the backmetal layer of the first solar cell. The methodfurther includes interconnecting the first solar cell and the secondsolar cell by coupling the exposed top surface of the backmetal layer ofthe first solar cell to a top surface of the second solar cell using aninterconnect tab.

According to an embodiment of the present invention, a solar cell arraycomprises a first solar cell and a second solar cell provided adjacentto the first solar cell. The first solar cell is formed of asemiconductor layer provided on top of a backmetal layer. A portion ofthe semiconductor layer of the first solar cell is etched to expose aportion of a top surface of the backmetal layer of the first solar cell.The first solar cell and the second solar cell are interconnected usingan interconnect tab by coupling the etched portion of the top surface ofthe backmetal layer of the first solar cell to a top surface of thesecond solar cell.

According to another embodiment of the present invention, a method forforming an array of solar cells includes providing a plurality of solarcells. A solar cell includes a semiconductor layer formed on top of abackmetal layer leaving a portion of a top surface of the backmetallayer exposed. The method also includes applying an adhesive to a filmlayer and placing the plurality of solar cells onto the film layer. Theplurality of solar cells are interconnected using one or moreinterconnect tabs by coupling the exposed top surface of the backmetallayer of a first solar cell to a top surface of an adjacent second solarcell. An adhesive is applied on the plurality of interconnected solarcells. The method further includes laminating a top film to theplurality of interconnected solar cells to form an array of solar cells.

BRIEF DESCRIPTION OF THE FIGURES

These and other characteristics of the present application will be morefully understood by reference to the following detailed description inconjunction with the attached drawings, in which:

FIG. 1 is a cross-sectional view of a conventional solar cell;

FIG. 2A is a cross-sectional view of a conventional solar cell arrayassembled according to conventional methods, and in particularillustrating the interconnect tabs contacting the top and bottom surfaceof adjacent solar cells;

FIG. 2B is a top view of the conventional solar cell array illustratedin FIG. 2A;

FIG. 2C is a bottom view of the conventional solar cell arrayillustrated in FIG. 2A;

FIG. 2D is a side view close-up of an exemplary conventionalinterconnect tab used in connection with the conventional solar cellarray illustrated in FIGS. 2A-2C;

FIG. 3 is a cross-sectional of an exemplary solar cell with etchedopening to expose the backmetal layer according to the teachings of thepresent invention;

FIG. 4A is a side view of an exemplary solar cell array connected usingthe interconnect scheme in accordance with the teachings of the presentapplication;

FIG. 4B is a top view of the exemplary solar cell array illustrated inFIG. 4A;

FIG. 4C is a bottom view of the exemplary solar cell array illustratedin FIG. 4A;

FIG. 4D is a side view close-up of an exemplary interconnect tab used inconnection with the exemplary solar cell array according to an exemplaryembodiment of the present invention;

FIG. 5 is a schematic flow chart diagram depicting the steps forassembling a solar cell array using the interconnect scheme according tothe teachings of the present application; and

FIG. 6 is a schematic flow chart diagram depicting the steps for formingan array of solar cells according to the teachings of the presentinvention.

DESCRIPTION

The present inventors have realized that for high-volume assembly ofsolar cell arrays, where the total annual production may exceed 100,000square meters per year, it is advantageous to perform all theinterconnect tabbing and bypass diode connections solely from a singleside of a solar cell array, i.e. either from the front or back surfaceof the solar cell array. Examples of high volume assembly of solar cellarrays include, but are not limited to, applications such as unmannedaerial vehicles, satellites, solar blankets, and solar power generation.During a high volume assembly of solar cell arrays, a plurality of solarcells may be picked and placed with precision onto a film layer using anautomated tool having optical alignment. Adjacent solar cells may beinterconnected from the same surface, either the top surface of thebottom surface, of the solar cells. The film layer may be part of thefinal assembled solar cell array or, alternatively, the film layer maybe a temporary carrier medium for assembly purposes. If the film layeris a temporary carrier medium for assembly purposed, the film layer maybe removed after the solar cells are assembled into a solar cell array.

According to an exemplary embodiment of the present invention, aplurality of the solar cells may be laid out to form a solar cell array.Interconnections between adjacent solar cells and bypass diodes may beattached or connected to the plurality of solar cells only from onesurface of the solar cell array, e.g. from the top side of the solarcell array. The foregoing manufacturing technique eliminates the need toaccess to the opposite surface, e.g. a back surface of the backmetallayer, of the solar cells. Once the interconnections and bypass diodesare placed and secured to the solar cells, a top sheet material may belaid across the solar cell assembly and secured in place.

As used herein, a top surface is the surface of the solar cell thatfaces, i.e. is directly exposed to, the light. As used herein, a bottomsurface is the surface of the solar cell opposite to the top surface.The bottom surface faces away from the light.

The assembly technique of the present invention requires the solar cellsto have a certain form. That is, semiconductor material in areas whereinterconnect tabs are to make contact with the backmetal of the solarcell are etched to expose a portion of the top surface of the backmetallayer as illustrated in FIG. 3. An epitaxial liftoff (ELO) methodologymay be used to facilitate removing the semiconductor material to exposethe backmetal layer according to various embodiments of the presentinvention. The ELO methodology involves removing or lifting epitaxiallayers that form the solar cell device with an attached metal back layerfrom a substrate with the aid of a release layer. Once released,standard semiconductor fabrication processes can then be performed toetch portions of the epitaxial layers thereby exposing the backmetal.This etching process can be performed using chemical wet or dry etchingprocesses since the epitaxial layers are relatively thin in the ELOapproach. For conventional solar cells grown on bulk substrate materialssuch as Germanium, the removal of the active semiconductor material inorder to expose the conductive back layer is more challenging becausethe active semiconductor layer stack is much thicker compared to the ELOapproach.

An exemplary solar cell 300 formed according to teachings of the presentapplication is illustrated in FIG. 3. The exemplary solar cell 300includes a semiconductor layer 306 formed on top of a backmetal layer310. A plurality of grid lines 304 may be formed on a top surface of thesemiconductor layer 308. A portion of the semiconductor layer 306 isetched to expose a top surface of the backmetal layer 310. As a result,the backmetal layer 310 of the exemplary solar cell 300 according tovarious embodiments of the present invention includes an exposed etchedsurface 308.

As illustrated in FIG. 3, the solar cell 300 formed according to theteachings of the present invention includes a top surface that faces thelight. The solar cell 300 includes a back surface that faces away fromthe light. Specifically, the top surface of the solar cell is the topsurface of the semiconductor layer 306 that faces the light. The bottomsurface of the solar cell is the bottom surface of the backmetal layer310 that faces away from the light. The bottom surface of the solar cell300 is opposite to the top surface of the solar cell 300. The solar cellalso includes an exposed etched surface 308 that exposes a top surfaceof the backmetal layer 308 of the solar cell 300. The top surface of thebackmetal layer 308 of the solar cell 300 is opposite to the bottomsurface of the backmetal layer 308 of the solar cell 300. That is, thetop surface of the backmetal layer 308 of the solar cell is adjacent andin physical contact with the bottom surface of the semiconductor layer306 of the solar cell 300.

A plurality of adjacent solar cells 300 with exposed etched surfaces 308may be electrically connected in series and/or in parallel. Allconnections, including diode connections and interconnections betweenadjacent solar cells, may be made solely from one surface of the solarcell array as shown in FIGS. 4A-4D. The assembly method described hereineliminates connecting interconnect tabs to the front and back surface ofadjacent solar cells as depicted in FIGS. 2A-2D in accordance with theconventional assembly techniques.

As illustrated in FIGS. 4A-4D, interconnect tabs 406 electrically bridgeadjacent solar cells 300 in series or in parallel by connecting the topsurface, e.g. a top grid line, of one solar cell to the top surface ofthe backmetal layer of an adjacent solar cell.

FIG. 4A is a side view of an exemplary solar cell array 400 connectedusing the interconnect scheme in accordance with the teachings of thepresent application. As illustrated in FIG. 4A, adjacent solar cells 300and 301 may be connected by forming a top or front connection 402 on theexposed etched surface 308 formed on the backmetal layer 310 of a firstsolar cell 300 and by forming another top connection 404 on the topsurface of the semiconductor layer 306 of an adjacent second solar cell301. As such, the interconnect tab 406 is connected to the top surfaceof the backmetal layer 310 of the first solar cell 300 and to the topsurface of the semiconductor layer 306 of the adjacent solar cell 301.

FIG. 4B is a top view of the exemplary solar cell array 400 illustratedin FIG. 4A. FIG. 4C is a bottom view of the exemplary solar cell array400 illustrated in FIG. 4A. FIGS. 4B-4C illustrate diode 409 attached tothe etched surface 308 of the backmetal layer from top front surface ofthe solar cell 300. One surface of the diode 409 is connected to thebackmetal through an etched opening 308 in the epitaxial solar cellmaterial. The opposite surface of the diode 409 is attached to a bypassdiode tab 408 that in turn is connected to the top surface of theadjacent second solar cell 301 to form the bypass connection. Thisconnection scheme is effectively able to bypass the first solar cell 300when necessary without the need for wrap around bypass diode tabs asseen in conventional solar cells. The flexibility of this approach alsoallows adjacent solar cells to be formed into either serial or parallelarrays to support either higher voltage or higher current applications,as required.

FIG. 4D is a side view close-up of an exemplary interconnect tab 406 anda bypass diode tab 408 used in connection with the exemplary solar cellarray according to an exemplary embodiment of the present invention. Asillustrated in FIG. 4D, the interconnect tab 406 is connected to the topsurface of the first solar cell 300 via the top connection 402. Theinterconnect tab 406 is also connected to the top surface of theadjacent solar cell 301 via the top connection 404. Specifically, theinterconnect tab 406 is connected to the exposed etched surface 308formed on the top surface of the backmetal layer 310 of the adjacentsolar cell 301. As further illustrated in FIG. 4D, the bypass diode tab408 is connected to the bypass diode 409 provided on the top surface ofthe solar cell 300. The bypass diode tab 408 is also connected to thetop surface of the adjacent solar cell 301. The bypass diode tab 408 isconnected to an exposed etched surface 308 of the adjacent solar cell301.

The solar cell assembly technique described herein may be used with aplurality of solar cell types, including but not limited to, singlejunction solar cells, dual junction solar cells, and multi-junctionsolar cells such as inverted metamorphic multi-junction (IMM) solarcells, etc.

Exemplary steps of the assembly method for forming solar cell arraysaccording to the various embodiment of the present application areillustrated in the flowchart 500 of FIG. 5. The method includesproviding a first solar cell (step 502). The first solar cell may be aconventional solar cell including a semiconductor layer provided on topof a backmetal layer. A portion of the semiconductor layer of the firstsolar cell is etched to expose a portion of the top surface of thebackmetal layer (step 504). A second solar cell is provided adjacent tothe first solar cell (step 506). The second solar cell may be aconventional solar cell or, alternatively, the second solar cell may beformed according to the teachings of the present application to includean exposed etched backmetal surface. If the second solar cell is notconnected to another solar cell other than the first solar cell, thesecond solar cell does not need to have an exposed etched surface. Thetop surface of the exposed backmetal layer of the first solar cell isconnected to the top surface of the second solar cell using aninterconnect tab (step 508). The top surface of the second solar cellmay include one or more grid lines. The top surface of the exposedbackmetal layer of the first solar cell may be connected to a grid lineprovided on the top surface of the second solar cell.

FIG. 6 illustrates a flowchart 600 describing the steps for forming asolar cell assembly using the connection method of the presentapplication. A plurality of solar cells with exposed backmetal portionsare provided (step 602). As described herein, a solar cell may be formedto include an etched surface on the top surface of the backmetal layer.An adhesive is applied to a film layer to secure the plurality of solarcells on the film layer (step 604). The plurality of solar cells isplaced onto the film layer, for example, to form a solar cell array(step 606). Bypass diodes are attached and/or connected to the solarcells from the top side of the solar cells (step 608). The bypass diodesmay be attached to the solar cells by welding or any other appropriatemethod. The interconnect tabs are attached to the solar cells from thetop side of the solar cells (step 610). The interconnect tabs may beattached to the solar cells by welding or any other appropriate method.Adhesive is applied on the assembly (step 612). Top film is laminated tothe assembly to secure the plurality of solar cells in place (step 614).The top film and the adhesive must have the required properties fortransmitting maximum light intensity and spectrum to the underlyingsolar cells.

The assembly method for forming solar cell arrays described hereinenables fast and efficient high-volume manufacturing since the bondingtool only needs to access the solar cell array from one single side.This also reduces handling of the solar cell arrays, reduces thepossibility of damage, which, in turn, increases the yield of solarcells from the fabrication and assembly processes.

Numerous modifications and alternative embodiments of the presentapplication will be apparent to those skilled in the art in view of theforegoing description. Accordingly, this description is to be construedas illustrative only and is for the purpose of teaching those skilled inthe art the best mode for carrying out the present application. Detailsof the structure may vary substantially without departing from thespirit of the present application, and exclusive use of allmodifications that come within the scope of the appended claims isreserved. It is intended that the present application be limited only tothe extent required by the appended claims and the applicable rules oflaw.

It is also to be understood that the following claims are to cover allgeneric and specific features of the invention described herein, and allstatements of the scope of the invention that, as a matter of language,might be said to fall therebetween.

1. A method for interconnecting a first solar cell adjacent to a secondsolar cell in an array of solar cells, the method comprising: providinga first solar cell adjacent to a second solar cell, wherein at least thefirst solar cell includes a semiconductor layer formed on top of abackmetal layer; etching a portion of the semiconductor layer of thefirst solar cell to expose a portion of a top surface of the backmetallayer of the first solar cell; and interconnecting the first solar celland the second solar cell by coupling the exposed top surface of thebackmetal layer of the first solar cell to a top surface of the secondsolar cell using an interconnect tab.
 2. The method of claim 1, furthercomprising: forming a metalized portion on the top surface of the secondsolar cell, wherein the exposed top surface of the backmetal layer ofthe first solar cell is coupled to the metalized portion formed on thetop surface of the second solar cell.
 3. The method of claim 1, furthercomprising: forming a release layer on a substrate; forming an array ofsolar cells on the release layer, the array including at least the firstsolar cell adjacent to the second solar cell; removing the release layerbetween the substrate and the array of solar cells; and separating thearray of solar cells from the substrate.
 4. The method of claim 1,wherein the first solar cell and the second solar cell areinterconnected from a same side of the first solar cell and the secondsolar cell.
 5. The method of claim 1, wherein the first solar cell andthe second solar cell are interconnected from a top side of the firstsolar cell and a top side of the second solar cell.
 6. The method ofclaim 1, wherein the first solar cell and the second solar cell areinterconnected by coupling a top side of the first solar cell to a topside of the second solar cell.
 7. The method of claim 1, whereininterconnecting the first solar cell and the second solar cell forms aserial connection between the first solar cell and the second solarcell.
 8. The method of claim 1, wherein interconnecting the first solarcell and the second solar cell forms a parallel connection between thefirst solar cell and the second solar cell.
 9. The method of claim 1,wherein interconnecting the first solar cell and the second solar cellforms a parallel connection and a serial connection between the firstsolar cell and the second solar cell.
 10. The method of claim 1, furthercomprising: connecting a bypass diode to a portion of the top surface ofthe second solar cell, wherein the first solar cell and the second solarcell are interconnected by coupling the exposed top surface of thebackmetal layer of the first solar cell to the bypass diode of thesecond solar cell.
 11. The method of claim 10, wherein the first solarcell and the second solar cell are interconnected from a same side ofthe first solar cell and the second solar cell.
 12. The method of claim10, wherein the first solar cell and the second solar cell areinterconnected from a top side of the first solar cell and a top side ofthe second solar cell.
 13. The method of claim 1, wherein theinterconnect tab is a metallic interconnect tab.